For the sake of clarity, here below referring to FIG. 1, a definition is provided of the terminology used in the description, especially a definition of the term “resource units” in an OFDM frame.
More specifically, the horizontal axis 11 defines the time indices of the OFDM symbols (symbol time) and the vertical axis 12 defines the frequency indices of the OFDM symbols (sub-carriers). A time-frequency symbol therefore corresponds to a given symbol time 111 for a given sub-carrier 121 and a resource unit 13 corresponds to a set of time-frequency symbols.
Thus, FIG. 1 illustrates a representation of the resource units (also denoted as “URs”) in an OFDM time-frequency frame, these resource units being located in frequency, i.e. using adjacent sub-carriers. Indeed, it may be recalled that the URs can be distributed or localized in frequency and may or may not use an entire time frame.
More specifically, we consider an OFDM type mobile network (for example a WIMAX or Worldwide Inter-Operability for Microwave Access type network) for which a time-frequency sub-division into disjoined resource units has been predefined. Such a network is presented especially in the document: “Part16: Air Interface For Fixed Broadband Wireless Access Systems”, IEEE Computer Society, October 2004 and corresponds to a short-term communications system based on an OFDM physical layer as described in the document “Physical layer aspects for evolved Universal Terrestrial Radio Access (UTRA)”, 3GPP Technical Specification Group Radio Access Network (R7), 3GPP TR25814 v7.0.0 (June 2006).
We also consider a cellular network comprising at least one base station and several mobile terminals associated with the base station or stations, each of these terminals having limited transmission power and several of these terminals seeking to transmit data at the same point in time.
Present-day techniques for allocating resources seek to optimize data transmission on uplink paths by adjusting the following parameters:                the number of resource units;        the type of modulation and encoding (also denoted as MCS for modulation and coding scheme);        the position of the URs in the time-frequency frame;        the transmission power allocated to the URs;        
in order to maximize the cell capacity of the system.
Classically, these prior-art techniques rely on the sharing between the different terminals of the network (also called mobile units or apparatus) of the time and frequency resource. Thus, the mobile units regularly send information back to the base station associated with them on the quality of their propagation channel and their needs in terms of bit rate. A base station may also itself make measurements of radio quality on the uplink and use the set of previous information elements to choose the number, position and transmission power of the resource units in the time-frequency frame. The base station then deduces the transportation schemes associated with each mobile unit as a function of the type of modulation and encoding scheme (MCS) chosen, i.e. according to the principle of modulation and encoding adaptation (or “link adaptation”).
Regularly, the base station sends its choice of transmission parameters to the mobile units present in the cell that it is managing. The mobile units can then transmit data elements in complying with the allocation set by the base station on which they are dependent.
More specifically, there is deemed to be a list of transport formats supported by mobile units, i.e. formats that can be transmitted by a mobile unit, a transport format being defined as the combination of a type of modulation and a type of encoding (MCS) with a fixed number of resource units to which there is a corresponding number of information bits (after decoding). A list of transport formats of this kind supported by a mobile unit is also called an authorized list.
If we consider a list of transport formats supported by a mobile unit and a choice of the size of the frequency band used by the mobile unit, it is thus possible to determine a sub-list of transport formats that the mobile unit can use.
Then, the mobile unit makes a selection from this sub-list of a transport format enabling the transmission of a maximum number of data elements with a given target quality (“link adaptation”).
Referring to FIG. 2, a more precise description is provided of a technique for the selection of a transport format to allocate the time-frequency resources as proposed in the document “System Analysis for UL SIMO SC-FDMA” (3GPP TSG-RAN G1#45/R1-061525, Shanghai, China, 8-12 May, 2006, Qualcomm Europe).
To this end, we consider a base station managing a plurality of mobile units (M mobile units). B denotes the total bandwidth of the OFDM system and Ns denotes the number of associated sub-carriers.
In this technique, the frequency band Bi usable by a mobile unit i is fixed and defined by:
            B      i        =                  P        max                    PSD        /        CL              ,with:                Pmax is the maximum transmission power available for the mobile considered;        PSD is the target spectral density of power of the mobile at reception; and        CL is a variable taking account of the propagation losses and antenna gain between the base station and the mobile unit.        
For each mobile unit i, the sub-list associated with it is formed on the basis of a list of transport formats supported by the mobile unit in retaining only the transport formats for which the occupied or busy frequency band is equal to Bi.
The result of this is that for a given target spectral density in reception, the total bandwidth used is very great (the greatest possible) and the number of mobile units transmitting simultaneously is the smallest possible.
In other words, each mobile unit M1, M2, . . . , MN has a maximum number of sub-carriers available (for example Δmax is the greatest possible value for the mobile unit M1), for a given target spectral density of power. Thus, if we consider M mobile units present in a cell managed by the base station, N mobile units only could simultaneously transmit data according to this technique, with N being smaller than or equal to M, and N being relatively small.
Furthermore, the number of sub-carriers N0 occupied for the transmission will be high.
It is thus observed that for a given mobile unit, the sub-list thus constituted leads to a use of a large part of the total frequency band of the system and to a small number of mobile units transmitting simultaneously. The prior-art techniques therefore seek to occupy the entire frequency band available for transmissions in the OFDM time-frequency frame.
Other techniques for choosing the size of the frequency band used by the mobile units of the network have also been envisaged by Yoon et al. (“Exploiting channel statistics to improve the average sum rate in OFDMA systems”, Vehicular Technology Conference, 2005, IEEE 61, 30 May-1 Jun. 2005, volume 2, pages 1053-1057) and Sternad et al. (“Channel estimation and prediction for adaptive OFDMA/TDMA uplinks, based on overlapping pilots”, Acoustics, Speech, and Signal Processing, 2005, Proceedings, IEEE International Conference on, Volume 3, 18-23 Mar. 2005, pages iii/861-iii/864).
In these two techniques, the bandwidth that can be used by a mobile unit of the network is fixed and respectively equal to B/N (where N is an arbitrarily fixed number common to all the mobile units, and is often a small number) and B. The result of this is that the total frequency bandwidth used is B and the total number of mobile units transmitting simultaneously is respectively equal to N, or 1.
According to these different techniques of the prior art, the selection of the transport format associated with a mobile unit of the network then consists in selecting the transport format that can be used to transmit the maximum number of data to be transmitted for a given target quality, as a function of radio conditions and data to be transmitted, according to the classic principle of link adaptation.
However, these prior-art techniques generate two main drawbacks, namely extensive cell interference and inefficient use of the transmission capacity of the mobile units.
Indeed, the fact of using a large part of the total bandwidth of the transmission system causes maximum inter-cell radio interference, thus limiting the cell capacity of the network considered.
Furthermore, the fact of spreading the transmission power of a mobile unit in the frequency domain over a wide bandwidth has the consequence wherein very few mobile units can transmit data at the same time. The result of this is that the maximum capacity of transmission of all the mobile units is not exploited.